In recent sound applications, often headphones are used which perform an active noise control or active noise cancellation. To this end, a microphone is situated at or inside the headphone providing a microphone signal comprising, for example, environmental noise. The active noise control evaluates the microphone signal and generates a correction signal provided to the loudspeakers of the headphone for minimizing the effect of the environmental noise or other disturbing sounds. For example, the external environmental noise and the correction signal cancel each other out, in an ideal case. However, microphones which are used in active noise control headphones usually have different gain characteristics due to manufacturing tolerances. Furthermore, the industrial design of the headphones and the mechanical implementations usually have different passive attenuation characteristics. Therefore, a gain of the noise control electronics needs to be adjusted during manufacturing of the headphone. In other words, active noise control headphones require calibration of gains at the production line. In particular, gain adjustment of the sensing microphones' amplification stages is required in order to achieve a wanted noise reduction performance. Such a calibration process is usually performed by adjusting of trimmer potentiometers that are manually set in the production line on a per headphone unit basis. To this end, the headphone is usually placed on an artificial head which includes microphones inside artificial ears for measurement purposes. An external test audio signal is generated to simulate environmental noise. An amount of residual test audio signal that arrives at the artificial ear after attenuation by means of the active noise control is measured. Individual microphone gains and other parameters of the active noise control circuit are adjusted manually in order to minimize the amount of test audio signal that is picked up by the measurement microphones. However, this calibration method results in increased manufacturing time and therefore increased costs.
Furthermore, if the active noise control circuit is located in the headphone itself, then the respective ear cup needs to provide an opening in order to enable access to the trimmer potentiometers. These openings may alter the acoustic behavior of the headset resulting in a less accurate calibration.
GB 2445984 A shows ambient noise reduction for ear-worn devices and the like. The suggested feed-forward noise-reduction system provides predetermined filter parameters, such as the gain and cut-off frequency of a selected filter stage used in the noise-reduction processing. These parameters are mathematically modelled and the model is adjusted in real-time, in response to user-interpretation of a graphical display of a predicted residual noise amplitude spectrum. This allows the user or computer control to inspect the predicted residual noise level amplitude spectrum and to iteratively adjust the filter parameters to minimise residual noise in a chosen environment.
U.S. Pat. No. 6,118,878 A shows an active noise cancellation system including several features such as locating a residual microphone radially offset from the center of a sound generator to detect a signal more similar to that incident upon the eardrum of the user and a mechanism for detecting changes in the acoustic characteristics of the environment The system automatically responds to changes, for example, by reducing the gain or the frequency response of the system to preserve stability. The system includes further features for detecting imminent instability and compensating for it.